zepp ant. with 450 ladder line, rf in shack?

[WB3DEH]

Have the following set-up.  A zepp cut for 20 mtrs, open wire feed line commercial 450 black ladder line to a MFJ-Versa tunerIII, mfj 961, connected to the balanced line input.

---------------- ------------------
     44'        !!       44'
                !!
                !!about 40' high drop straight down.(450ohm)
                  20' fed direct to tuner in shack.(450ohm)
  rod/grnd -----tuner----radio---mike, MC-80

Loads fine on all bands with the tuner, vry low swr on all <1.2:1 when tuned...

Problem:  I use a Kenwood ts430 with a MC-80.  It appears that at hf fequencies greater than 14mhz...I.e. the 21mhz,28mhz area I am experiencing problems with the MC-80 microphone (appears to be a amplified mike, I have no data sheet) It appears that RF is getting into the mike, since the MFJ versa tuner and 430 are only about 2'-4' away.  MC-80 works fine on the lower frequencies from 14mhz down to 3.5mhz. I switched mikes, to the 430 mobile version,
MC42s and have no problems at any band (this is a none amplified mike--push to talk)  So it appears to be RF or an ocillation in the MC-80,  my guess is RF...Is there a better solution to the above feed system that I could use and still use the 20mtr zepp for all band use? (to cut down on the RF in the Shack)  Also, I have the station firmly grounded, 8' ground rod, with 1/2" braided copper wire firmly attached to 430 and tuner.  Any ideas on RFI fixes for mike mc-80 or Feed system....

Tnx for any feedback in advance..

Bob    RobertSteele@digitalexp.com

[WB6BYU]

It may just require a bypass capacitor across the input to the
internal amp and/or on the DC lead.  I have the MC-60, which
has a switch on the bottom to bypass the internal amp (and disconnect the internal battery.)  You might also make sure the speech processor
is turned off.

As far as the antenna goes:  it doesn't look like you are using the
original "Zepp" (which only connected to one side of the feedline)
but are simply feeding a doublet with 450 ohm line.  Presuming the
balun is connected properly, you shouldn't have much trouble with
RF in the shack.  But it seems that you do.  My first suggestion is to
add a quarter wave radial wire to the ground lug on the tuner.  Try it
on one band, and, if it makes a difference, add additional wires for the
other bands.  (It may be that wires for 2 or 3 bands clean up the
others.)

A common problem with ground rods is the length of the wire connecting
the rod to the station.  If this is on the order of a quarter wavelength long,
(which is only 8' on 10 meters) it will act as an impedance transformer,
just like a quarter wave of feedline.  If you short one end, the other end
looks like an open circuit, which doesn't do a good job of grounding the
rig.

It is also possible that the shield on your mic wire is not properly
grounded at both ends.

Good luck!   - Dale  WB6BYU

[AC5UP]

You may be able to RF-proof the mic by adding bypass caps and ferrite beads, but that won't solve the problem of RF in the shack. (and the phone, TV, stereo, etc.)

There are situations when the antenna and feed line combo on some bands can end up on the ragged edge of the tuners' range, and you can tweak that by changing the length of the feed line. Add a foot or two of scrap line to the feeder or trim a little slack to see what happens.

This will change your tuner settings, and that's the idea.

Could make it better, could make it worse, or move the RFI to another band. If nothing changes, you'll have a better idea of what's not part of the problem.

[VE7GL]

I have a similar situation with an Extended Double Zepp for 80 meters being fed with 130’ of 450 ohm ladder line.  How I solved it was by adding about 10’ of paralleled RG213 50 home coax with the center conductor going to each side of the 450 on one end and to the tuner’s balanced output on the other.  Both ends of the coax were grounded.  This allowed me to leave the ladder line outside the shack, away from the radio equipment and anything else that might unbalance the ladder line causing it to radiate.  

As long as the coax run is fairly short there shouldn’t be an issue with losses or impedance transformation issues.  I’ve tested it with 1KW DC output on 160-40 meters, no problem.

Dale J.
VE7GL
http://reboot.bc.ca